| 摘要 |
PURPOSE:To improve the Ftheta characteristic (positional deviation specification on a scanning surface), curvature-of-image-field characteristic, etc., by using a combined lens as the 1st lens (lens close to rotary polygon mirror) of an Ftheta lens system. CONSTITUTION:The combined lens which is bisected is used as the 1st lens 51 of the rotary polygon mirror 10. The Ftheta lens system 1 consisting of the 1st lens 51 and a 2nd lens 52 converges the light beam, which is smoothed in a Y direction by a cylinder lens 3, in an X direction (main scanning direction) to scan a scanning plane 16. Further, a reflecting surface 40 and the scanning plane 16 are in geometrical conjugation relation in the Y direction (subscanning direction). Thus, the combined lens is used as the 1st lens 52 to increase the number of refracting surfaces to five, so the degree of freedom of design of each of the curvature, surface interval, refractive index, etc., of the Ftheta lens system 1 is increased by one. Consequently, the design can be made finer, the curvature of field is suppressed within the depth of focus, and >=5 distortion aberrations can be compensated at the same time. |
